The present invention relates generally to integrated metal oxide semiconductor (MOS) devices and, in particular, to combining high voltage and low voltage devices on a single semiconductor substrate.
Refinements in the design and fabrication of integrated circuits (ICs) have provided circuits having more than one million transistors. With continued research and development, the microelectronics industry continues to push transistor densities even higher. One consequence of increasing transistor densities is that device operating voltages are generally getting lower. Whereas, older transistor-transistor logic (TTL) circuits were designed to operate at 5 volts (v), newer circuits such as memory devices and central processing units (CPU) are being designed to operate at 3.3v, 2.5v, and 1.8v.